Implemented primitive optional parameter support for external functions

* renamed FunctionExternalTextFormatter into FunctionExternalCommand
** the goal is to provide a general interface to a variety of external commands
*** e.g. not just text formatters but system utilities for file management and so on
** this requires the stdin parameter to be optional as not all external commands require stdin input
* implemented "filter_derived" helper template to determine amount of optional parameters
** optional parameters are defined as "boost::optional" specializations
*** they in turn can be detected by checking if "boost::optional_detail::optional_tag" is a base class
* "callConstructDocument" member method of "FunctionBase" performs additional bounds checking of parameter vector
* "boost::optional<std::string>" specific member overload was added to "XObjectValue" helper class
** we will have to provide full specializations for all optional types as C++ prohibits partial member function template specialization
* renamed the external function in "PlattformGuard"
* changed README.md and test cases accordingly

1 files changed, 2 insertions, 2 deletions

diff --git a/ixslt.cc b/ixslt.cc
index 23dc9ca..ff5de25 100644
--- a/ixslt.cc
+++ b/ixslt.cc
@@ -2,8 +2,8 @@
 #include <xalanc/PlatformSupport/XalanStdOutputStream.hpp>
 #include <xalanc/XMLSupport/FormatterToXML.hpp>
 
-#include "boost/optional.hpp"
-#include "boost/program_options.hpp"
+#include <boost/optional.hpp>
+#include <boost/program_options.hpp>
 #include <boost/filesystem/fstream.hpp>
 
 #include <string>